X-ray diffraction analysis uses X-ray energy focused on a target material of interest to determine structural qualities of the material. For a particular material, and X-rays at a particular wavelength, the X-ray energy is diffracted and forms a diffraction pattern that may be detected with a detector to yield information regarding the material structure. Conventional X-ray sources are able to generate only a single wavelength as they rely on characteristic Kα radiation from a particular target material. That is, these sources function by illuminating a material target with a high energy electron beam. This excites both continuum Bremsstrahlung radiation and also characteristic line emission. For most analytical applications, a monochromator is used to isolate only the characteristic line emission.
Particular X-ray energies may be generally preferable for certain applications. For example, for small molecules, highly absorbing samples, charge density measurements or for work with diamond anvil cells, relatively short wavelength Mo radiation (λ=0.73 Å) is most often employed. For protein crystallography or determining the absolute structure of light-atom organic molecules, a longer wavelength is preferred (in most cases, copper with λ=1.54 Å). In some other specialized experiments, silver, chromium or cobalt might be preferred.
If one wishes to change the operating wavelength of an X-ray source, the target material of the source may be changed. However, this changeover procedure can be tedious and time-consuming. This is especially true for high-brilliance rotating anode generators. In order to change the target in a conventional rotating anode, the system must first be powered down (and the rotating of the anode stopped), the vacuum chamber must be opened, the anode must be disconnected, dismounted and replaced, the vacuum must then be reestablished, and the new anode must be restarted and reconditioned. After the target has been changed, the X-ray optics must also be replaced if one is using modern multilayer optics, which operate optimally only at a single fixed wavelength. Also, since the optics will have different take off angles, the goniostat will typically have to be repositioned and the optics thereafter realigned. This entire process can easily take from several hours to an entire day to complete.
In order to address this difficulty, sources have been proposed that may operate at two different wavelengths. U.S. Pat. No. 4,007,375 discloses a multiple wavelength X-ray tube in which one of several target materials may be selected by electrostatic deflection of an incident electron beam. A similar design shown in Japanese patent JP5135722 has a multiple wavelength X-ray source in which the tube consists of several tracks composed of different materials deposited on a rotating anode. One of the target materials is selected by deflecting the incident electron beam magnetically. In both of these patents, however, the associated monochromator optics would have to be changed and aligned, so there is no means by which a system could change immediately from operation at one wavelength to operation at another.
Japanese patent JP2848944 discloses a dual wavelength X-ray source which also uses an anode with two tracks composed of different respective target materials. In this case, however, the cathode filament is physically moved to change the wavelength. But the optics must also be changed and aligned, so that operation at the new wavelength is not instantaneous. This is also the case in Japanese patent JP11339703, which has multiple target materials and an electron gun that is rotated to select the wavelength.
U.S. Pat. No. 6,041,099 describes a side-by-side Kirkpatrick-Baez multilayer optic that is a multilayer monochromator and beam conditioning optic for the focusing of an X-ray beam onto a sample. This optic is comprised of two multilayer mirrors attached to each other at a relative angle of 90°, and it has a single corner in which two-dimensional beam collimation or focusing takes place. This arrangement, often referred to as a “Montel optic” according to its first mention, also appears in U.S. Pat. No. 6,014,423, in which a combination of such optics is described which has multiple corners, typically four, to reflect the X-rays, with the aim of enhancing the X-ray flux. The configuration allows for the reflection of radiation from a single anode to a sample position.
U.S. Pat. No. 6,421,417 describes a multilayer optic with adjustable working wavelength. Here, for a wavelength change, the optic either needs a change of the curvature, or the multilayer structure is configured to include a plurality of d-spacings, or the optic is formed with stripe-like multilayer coating sections. A change of the curvature requires a major realignment of the optic. The use of a plurality of d-spacings leads to a compromise where the performance is lower than the performance of two optics that are fully optimized for their individual working wavelengths, in addition to requiring a major realignment of the optic. Stripe-like multilayer coatings cannot be used with a Montel arrangement, and in a stripe-like multilayer, the optic has a single, fixed curvature. Therefore, the optics cannot be fully optimized for best performance at the different wavelengths.
U.S. Pat. No. 6,917,667 discloses a multilayer optic that can be used for two wavelengths, but for which the wavelength change requires a realignment of the optic. Further, the principle described in this patent functions only if the two wavelengths are close to each other, e.g., for Cu and Co radiation, because it neglects the effect of refraction, which is wavelength-dependent. Therefore, the principle of this patent functions only when the two wavelengths are appropriately selected. Since the optic of this patent is a compromise, the performance at the two wavelengths is reduced compared to optics that are fully optimized for a single working wavelength. Furthermore, the length of these optics is limited to typically 40 mm, leading to a rather small opening aperture and small capturing efficiency.